fix: Use posix path for dockerfile path passed to docker build API

[hnnasit]

Which issue(s) does this change fix?

#5345

Why is this change necessary?

When sam build is run for CDK projects on Windows for image-type functions with Dockerfile present in a nested directory, the command fails. Docker build API expects a posix path (with forward-slash) to be passed for dockerfile parameter but when sam build is run for CDK on Windows, Windows format path (with backslash) is passed causing build image to fail. Non-CDK projects currently passes posix path for dockerfile.

resource_metadata_normalizer.py is updated so that dockerfile path saved in build.toml is consistent with the value passed to docker API and with how non CDK templates saves this parameter.
app_builder.py is updated so that any new workflow calling build passes a posix path and does not break this behavior in Windows.

How does it address the issue?

Fixes the issue mentioned above

What side effects does this change have?

There should not be any side effects. Integration tests have been added to test the new behavior.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.